Impact of leading power factor
loads on synchronous alternators
by Gary Olson, Cummins Power Generation
Many electrical loads incorporate elements that can impose a leading power factor on the power source. While these loads are typically not
a problem for utility power sources, leading power factor can cause generator set failures or the failure of certain loads to operate properly
on a generator set.
The problems experienced when
attempting to operate generator sets
with leading power factor loads may
seem mysterious, but in reality, they are
not too different from another energy
absorption problem: the limited ability of
a generator set to absorb real kW power
from loads in certain applications. A
generator is physically unable to absorb
more than a very small amount of real
(kW) or reactive (kvar) power. While the
reverse kW power produced by a falling
load in a crane application may drive the
engine into over-speed conditions when
it exceeds the ability of the engine to
absorb it, the reverse kvar load presented
by leading power factor devices will drive
the alternator into over-voltage conditions.
O v e r t h e p a s t y e a r s, g e n e r a t o r s e t
manufacturers have evolved their
equipment designs to include the use
of digital automatic voltage regulators
(AVR), separate excitation systems, and
PWM-type control architecture to enable
the generator set to produce a stable
output voltage and successfully operate
non-linear loads. At the same time,
manufacturers of equipment that has
non-linear load characteristics have
begun to commonly employ filters to limit
harmonic current distortion induced on
the power supply. Capacitive elements
are also applied in facilities to improve the
power factor when operating on the utility
source to avoid higher energy charges.
While filters provide positive impacts on
the overall power system, they can be very
disruptive to generator operation.
A utility supply simply absorbs the reactive
power output because it is extremely large
relative to the filter system and it has many
loads that can consume this energy. With a
generator set, however, the rising voltage
from the leading power factor causes
the voltage regulator to turn down and
reduce alternator field strength. If the
AVR turns all the way off it loses control of
system voltage, which can result in sudden
large increases in system voltage. The
increase in voltage can result in equipment
damage and failure.
A UPS is designed to recognize high
voltage as an abnormal and undesirable
condition, so it can immediately switch
off its rectifier. When it does that, the high
voltage condition is immediately relieved
(because the filter is disconnected from
the generator set) and voltage returns to
normal. To the observer, the generator will
seem to be unable to pick up the system
load.
Paralleling problems
Generator sets that are used in isolated
bus paralleling systems have particular
issues with leading power factor loads.
When loads are applied to a parallel
generator bus, the total load on the
system can be many times larger than
the capacity of a single generator set. The
generator sets close to the bus one at a
time, so that if high loads (either leading
or lagging) are applied before genset
capacity is available, the generator bus
can fail. With leading power factor loads,
the failure mode will be due to either an
over-voltage condition or reverse kvar
shutdown. Further, there is a tendency,
particularly in data centre applications, to
group UPS loads together on a common
bus. This concentrates the leading power
factor load on one bus, so that if a large
UPS load is applied to the first generator
set available, it can easily be driven into
an excess reverse var condition, which
The AVR monitors generator output voltage
and controls alternator field strength
to maintain constant output voltage.
Relatively low AVR output is required to
maintain generator voltage at no load.
In Fig. 1, the no load exciter field current
required is less than half the full load level.
Filter equipment is often sized for operation
at the expected maximum load on the
UPS or motor load. At light loads there may
be excess filter capacitance, causing a
leading power factor. Since rectifiers are
commonly designed to ramp up from zero
load to minimize load transients, leading
power factor loads may be imposed on
the system until inductive loads are added
to the system or the load factor of the nonlinear load increases.
Fig. 1: In this example no load field required is 17 A, while full load is approximately 38 A.
energize - June 2013 - Page 17
GENERATION

loads may be required to be broken into
smaller blocks of UPS and mechanical
loads, rather than larger isolated buses
of each.
The filters should be turned off when
operating on the generator set or reduce
the magnitude of filtering provided. If
the generator is provided with modern
digital excitation control, the filters won’t
be needed to maintain stable generator
operation, but may still be required to
properly serve other loads.
Fig. 2: Green area is normal operating range of a typical synchronous machine, yellow is
abnormal but not damaging, and operating in red regional will cause damage or failure.
will result in over-voltage and shutdown. If
multiple generator sets are on the bus and
a large reverse var load is applied to the
genset bus, the var load sharing control
system can be disrupted, because not all
load sharing control systems include logic
for reverse kvar load sharing. If reverse kvar
load sharing is not in the control system’s
logic the system will typically cause one
or more generator set to exceed their
reverse power limits, which can cause
pole slipping.
Generator sets in a paralleling system
are maintained in synchronism by their
magnetic fields. When a leading power
factor load is applied, the voltage of
the genset or bus rises, and the voltage
regulation system of each generator
set reduces exciter power, reducing
the strength of the magnetic field. If
the field is switched off in an attempt
to reduce voltage to an acceptable
level, the generator set may slip a pole,
which results in potentially catastrophic
alternator damage. The reverse kvar limit
of the aggregated generators is the sum
of the reverse var limits of each generator.
However, the reverse var settings may
not be able to take advantage of all
the capability of the alternators due to
limitations in the var load sharing system.
Solutions
What can be done about this? First, we
need to understand how much reactive
power can be absorbed by the generator
without negative impact. The ability of an
alternator to absorb power is described
by a reactive capability cur ve. Fig. 2
shows a typical generator capability curve
describing the capability of a machine
to produce and absorb power. In this
curve the kvar produced or absorbed is
on the x-axis (positive to the right). The
y-axis shows kW (positive going up). kvar
and kW are shown as per unit quantities
based on the rating of the alternator (not
necessarily the generator set, which may
have a lower rating).
The normal operating range of a generator
set is between zero and 100% of the kW
rating of the alternator (positive) and
between 0,8 and 1,0 power factor (green
area on curve). The black lines on the
curves show the operating range of a
specific alternator when operating outside
of normal range. Notice that as power
factor drops, the machine must be derated to prevent overheating. On the left
quadrant, the near-normal output (yellow
area) can be achieved with some leading
power factor load, in this case, down
to about 0,97 power factor, leading. At
that point, the ability to absorb additional
kvar quickly drops to near zero (red area),
indicating that the AVR is “turning off ” and
any level of reverse kvar greater than the
level shown will cause the machine to lose
control of voltage.
In other words, if the machine is rated
for operation at 1000 kVA and 0,8 power
factor (600 kvar), with a reverse kvar level
of 0,2 per unit, the machine’s capabilities
will be exceeded. So, with more than
120 kvar reverse power and leading power
factor lower than 0,97 (for most people a
surprisingly low level) we have a problem.
The solution to this problem on this specific
machine involves avoiding excess reverse
kvar levels through proper system design
and operation.
The sequence of operation must be
modified so that loads that require reactive
power are present on the bus when
the UPS connects to the generator. The
reactive power produced by the filters will
be consumed by the system loads, and
the loss of voltage control is avoided.
This requires a re-thinking of operating
sequences in some cases because:

Mechanical loads rather than UPS may
need to go on the generator first.
energize - June 2013 - Page 18
The actual limits of reverse kvar can vary
considerably from machine to machine,
both within a specific manufacturer ’s
product line, and between equipment
from different suppliers. A good rule of
thumb for certain equipment is that it can
absorb about 20% of its rated kvar output
in reverse kvar without losing control of
voltage.
Alternators are physically limited in their
ability to both produce and absorb power.
When a leading power factor load is
applied to an alternator at a site, generator
failure, over-voltage, load failure, and
alternator damage can occur. There is
very little that the alternator supplier can
do to resolve problems at a specific site
other than to help a system designer
understand the nature of the problem and
the limits of the machine as installed. Most
of the solution will come from changes
in the system sequence of operation, or
hardware changes that prevent disruptive
reverse var conditions from affecting the
generator set.
Conclusions and recommendations






Synchronous alternators have limited
ability to absorb kvar from load
devices, and exceeding this limit will
result in generator set shutdown.
Paralleling operations require careful
consideration of the loading sequence
to prevent reverse var conditions that
can damage the generator set.
Consider modifying system sequence
of operation or limiting filter operation
until adequate loads are in place to
prevent reverse kvar conditions on the
genset.
Specify the magnitude of reverse kvar
the genset must be able to absorb, not
the power factor alone.
In single generator applications
protective devices can be set to the
limits of the alternator.
In paralleling applications both
alternator limits and var load share
limits must be considered.
Contact Dave Wardell,
Cummins, Tel 011 589-8428,
dave.wardell@cummins.com 